I have a G27 too and I am very happy with it but there are things I would want to be better (rattling, deadzone, slow turning speed, more precise pedals, feeling of the single cogs of the gears in the motor while holding a turn etc.).
I have an old Thrustmaster wheel which isn't really comparable but it was belt driven and a lot smoother and more quiet. In my opinion even a TX-PC would be a nice upgrade, but not worth the money in comparison.
The Fanatecs seem to be very good too. They are like the highest step before going direct drive but I read much about that they are "toys" when compared to a DD wheel.
In the end you have a few options:
Thrustmaster TX: good upgrade, but not a HUGE difference
Fanatec: really good upgrade, but also expensive
Accuforce: simple and complete package, very expensive, top DD wheel, but slightly worse FFB (some people say you feel the cogs in some situations) than OSW.
OSW: around the same price as Accuforce, better FFB, fully customizable, but less "put it on and go racing".
What I would recommend: You can buy all these wheels without pedals and the G27 pedals are not THAT good. I modded my pedals with sponges and that's already a big difference for brake control. Good pedals are really making a difference!
So I would recommend to first buy a really good set of pedals, look how much you can and want to spend for them and feel the first step to "high end" and then decide for the wheel.
If you buy one of those expensive wheels you will need to buy good pedals anyway
I can also recommend to watch this video (careful, loud music!):
LINK (youtube, pro drift, Assetto Corsa Nordschleife)
Look at how responsive that OSW is! When he lets go of the wheel it just spins instantaneous and when he grabs it, it gives so many little details. BUT: his footwork is amazing too!!